This invention relates to isolation amplifiers, and more particularly to circuitry and techniques for very precisely coupling the times of occurrence of pulse signals across isolation barriers, particularly low capacitance capacitive isolation barriers, to produce a lower cost, higher bandwidth isolation amplifier with higher input-to-output isolation voltage than has heretofore been available.
Isolation amplifers have been developed for various applications wherein considerably greater electrical isolation between amplifier inputs and outputs is required than can be obtained for conventional "instrumentation amplifiers" and differential amplifiers. Such "isolation amplifiers" are widely used in applications in the medical electronics field and in industrial and military applications wherein it is essential that there be minimum coupling of common mode signals from inputs of amplifiers to their outputs, for example, due to electrostatic discharge signals and electromagnetic pulse signals. Typically, in isolation amplifiers the original analog input signal has been modulated in a variety of ways, including amplitude modulation, frequency modulation, pulse width modulation, duty cycle modulation, and phase modulation. Considerable difficulty has been encountered by circuit designers attempting to obtain high frequency, low distortion performance at low cost and with high reliability for a variety of reasons, including the presence of nonlinearities and temperature dependency of the "isolation barrier" or intermediate transmission medium.
An "isolation" amplifier may be required, for example, to amplify an input AC signal having an amplitude of as low as a few millivolts superimposed upon a large transient common mode voltage as high as 1500 to 3500 volts, or even higher. Most prior isolation amplifiers have utilized magnetic transformers or optoelectronic devices as isolation barriers. However, the cost of isolation amplifiers using optoelectronic or magnetic isolation barriers has been quite high. Furthermore, the bandwidth of isolation amplifiers using optoelectronic or magnetic isolation barriers has been lower than is desirable. The present state-of-the-art is exemplified by the assignee's Burr-Brown ISO 100 optically coupled isolation amplifier, which has an isolation voltage of 750 volts, a bandwidth of 60 kilohertz, and a cost of roughly $30.00.
Although high performance isolation amplifiers have found an increasing market over the past few years, their cost is so high that most users build their own isolation amplifier circuits, because it is generally perceived by users that it is less expensive to manufacture a particular isolation amplifier that meets their requirements than to purchase a suitable commercially available "off-the-shelf" device. The assignee's marketing research indicates that if high performance isolation amplifiers having an isolation voltage of about 1500 volts or more and bandwidth greater than about 1 kilohertz could be manufactured economically, for example, for less than about $10.00 to $15.00, there would be a large market for such devices. However, until now it has not been possible to provide such an isolation amplifier, despite intensive efforts being directed by major suppliers of hybrid integrated circuits toward this objective.
It is clear that there is an unmet need for an improved low cost, high bandwidth, highly reliable isolation amplifier having an isolation voltage of about 1500 volts or more.
The state-of-the-art is believed to be indicated by commonly assigned U.S. Pat. No. 4,292,595 (Smith) and U.S. Pat. No. 3,714,540 (Galloway). The isolation amplifier in Pat. No. 4,292,595 introduces the idea of using a pair of capacitors as an isolation barrier in an isolation amplifier. The described circuit is quite complex, and the high voltage isolation barrier capacitors need to be very large, roughly 50 picofarads, making the circuit impractical. The circuit shown in U.S. Pat. No. 3,714,540 cannot operate in conjunction with a capacitive isolation barrier, as would be desirable in order to eliminate the high cost and slow speed of the prior optoelectronic and magnetic transformer isolation barriers used in most isolation amplifiers. In this reference, the signal coupled across the isolation barrier is differentiated with an RC time constant that allows the resulting pulse coupled across the isolation barrier to "droop" through an operational amplifier offset voltage, resulting in inaccurate triggering. Such inaccurate triggering results in inaccurate decoding and consequently inaccurate analog output signal levels. The foregoing references do not provide the guidance needed by one skilled in the art to make a low cost, reliable, isolation amplifier having high isolation voltage, high bandwidth, and wide dynamic range.